1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor, a method for manufacturing the photoreceptor, and an image forming apparatus and a process cartridge using the electrophotographic photoreceptor. More particularly, the present invention relates to an electrophotographic photoreceptor for use in electrophotographic image forming apparatus such as copiers, facsimiles, laser printers, and direct digital plate making machines, and a method for manufacturing the photoreceptor, and an image forming apparatus and a process cartridge using the electrophotographic photoreceptor.
2. Discussion of the Background
The photosensitive material for use in the photoreceptors for electrophotographic image forming apparatus such as copiers and laser printers has changed from inorganic photosensitive materials such as selenium, zinc oxide and cadmium sulfide to organic photosensitive materials. This is because organic photosensitive materials are friendly to environment, and have low manufacturing costs; and good designing flexibility.
Organic photoreceptors are broadly classified into the following three types:    (1) homogeneous single-layered photoreceptors in which, for example, a photoconductive resin such as polyvinyl carbazole (PVK) or a charge transfer complex such as PVK-TNF (2,4,7-trinitrofluorenone) is formed on an electroconductive substrate;    (2) dispersion type single-layered photoreceptors in which a resin layer including a pigment such as phthalocyanine and perylene dispersed in the resin is formed on an electroconductive substrate; and    (3) functionally-separated multi-layered photoreceptors in which a charge generation layer (hereinafter referred to as a CGL) including a charge generation material (hereinafter referred to as a CGM) and a charge transport layer (hereinafter referred to as a CTL) including a charge transport material (hereinafter referred to as a CTM) are overlaid on an electroconductive substrate.
The functionally-separated multi-layered photoreceptors typically have a structure in which a CTL is formed on a CGL. Functionally-separated multi-layered photoreceptors having a reverse structure are sometimes referred to as reverse-layered photoreceptors.
In particular, the functionally-separated multi-layered photoreceptors have advantages of having high photosensitivity and good flexibility in designing photoreceptors having high photosensitivity and good durability. Therefore, recently the functionally-separated multi-layered photoreceptors are widely used for electrophotographic image forming apparatus.
In recent years, it is very important to manufacture products while considering influence thereof on the global environment. Therefore, photoreceptors to be used in electrophotographic image forming apparatus are required to be one of mechanical parts instead of a supply (i.e., a disposable product). In other words, photoreceptors are required to have a long life. Therefore, recent photoreceptors typically have a protective layer located on a photosensitive layer.
On the other hand, as for the toner for use developing latent images formed by electrophotography, polymerizedtoners, spherical toners and small-sized toners (having an average particle diameter not greater than 6 μm) have been widely used because of being friendly to global environment and being able to produce high quality images. In order that photoreceptors have good cleanability against such toners, a protective layer is typically formed on the surface of the photoreceptors.
The photoreceptors having a protective layer thereon tend to cause problems in that a ghost of a previously formed image is formed in the following images (hereinafter referred to as a residual image problem or a ghost image problem). In other words, there is no photoreceptor which has good combination of durability, toner cleanability and image quality.
Particularly, in high speed image forming apparatus such that the interval between the light irradiation process and the following development process (hereinafter referred to as the exposure-development interval) is not greater than 100 msec, the residual image problem tends to occur. This problem hardly occurs in low- or medium-speed image forming apparatus. In the present application, high speed image forming apparatus are defined as an apparatus in which the exposure-development interval is not greater than 100 msec.
The residual image problem will be explained in detail.
When a half-tone image is formed following an image which has only light image portions and dark image portions as illustrated in FIG. 14, there is a case where a residual image (positive or negative image) of the image can be observed in the half-tone image as illustrated in FIGS. 15 and 16. These images are so called “a positive residual image” or “a positive ghost image” (illustrated in FIG. 15) and “a negative residual image” or “a negative ghost image” (illustrated in FIG. 16). It is necessary to prevent formation of such a residual image problem particularly in high quality full color image forming apparatus.
The mechanism of formation of a residual image is considered to be caused by fluctuation of the surface potential of the photoreceptor as discussed in published unexamined Japanese Patent Application (hereinafter JOP) 11-133825. The mechanism will be explained referring to FIG. 17.
FIG. 17A illustrates the surface potential of a photoreceptor after the photoreceptor is charged so as to have a potential of −700 V and then exposed to imagewise light (i.e., the surface potential of an electrostatic latent image formed on the photoreceptor). In this case, the surface potential of the lighted portion is about 0 V. FIG. 17B illustrates the surface potential of the photoreceptor after the latent image is developed with a toner (i.e., the surface potential of the photoreceptor having a toner image thereon). FIG. 17C illustrates the surface potential of the photoreceptor after the toner image is transferred onto a receiving paper while a reverse bias is applied to the receiving paper. In this case, the lighted portions have a certain positive potential (for example, +10 V in FIG. 17C).
When the photoreceptor is charged after these image forming processes are repeated, the surface potential (for example, −690 V) of the former image portion is lower than that (i.e., −700 V) of the other portions. If a half-tone image is formed on an area including the former image portion and a former non-image portion, the difference in potential between the former image portion and a non-lighted portion is larger than that between the former non-image portion and the non-lighted portion, and thereby a dense image (a positive image) is formed on the former image portion.
As discussed in JOP 2002-123067, the residual image problem is also caused in a digital image forming method in which half tone images constituted of digital dot images are formed similarly to the inkjet printing methods.
Specifically, the illuminance in a beam spot formed on a photoreceptor to form a latent dot image thereon is not uniform and has a certain distribution in a direction of from the center to the periphery of the beam spot. When a beam spot is formed on the former image portion, the resultant latent dot image portion has a larger area than the other latent dot image portions because the potential of the latent dot image portion is biased by, for example, +10 V. Thus, the resultant dot toner image has a large diameter than that of the dot image in other portions, and thereby the portions of the widened dot image potions are observed to be dense, resulting in formation of a residual positive image. This phenomenon is more prominent in high definition image formation with resolution of 1200 dpi than in image formation with resolution of 600 dpi.
As discussed in JOP 10-177261, the main reason for the fluctuation of surface potential is considered to be that space charges are stored in the inside of a photoreceptor. In attempting to prevent storage of space charges, the following methods have been disclosed.
(1) Improvement of Outermost Layer of Photoreceptor
JOP 10-115946 discloses a photoreceptor having an outermost layer which includes a polyarylate resin and which has a dielectric constant not less than 2.3.
JOP 11-184135 discloses a photoreceptor having a photosensitive layer including an azo pigment and an outermost layer including a polyarylate resin. According to the publication, polyarylate resins have high crystallinity, and therefore can orient the CTM included therein to some extent. It is considered that by orienting the CTM and using the specific azo pigment, the charge injection barrier can be decreased and thereby the photo-memory property of the photoreceptor can be diminished.
JOP 10-177263 discloses that a photoreceptor having a CGL including a phthalocyaine compound and an outermost layer including a bisphenol-based polycarbonate is used for an electrophotographic image forming apparatus having an intermediate transfer medium.
JOP 10-177264 discloses that a photoreceptor having a CGL including a phthalocyaine compound and an outermost layer including a charge transport polymer is used for an electrophotographic image forming apparatus having an intermediate transfer medium.
JOP 10-177269 discloses that a photoreceptor having a CGL including a phthalocyanine compound and either an insulative outermost layer or a semiconductive outermost layer including at least a resistance controlling agent is used for an electrophotographic image forming apparatus having an intermediate transfer medium.
JOP 2000-147803 discloses a photoreceptor in which a polycarbonate copolymer obtained from bisphenol A and a monomer having a specific arylene group is used for the outermost layer thereof such as the CTL. It is discussed in the publication that injection of charges having a reverse polarity from the outermost layer side can be prevented.
JOP 2001-235889 discloses a photoreceptor having an outermost layer including a surface-treated particulate metal oxide, an alcohol-soluble resin and an alcohol-soluble CTM. It is described in the publication that thermoplastic resins cannot be used as the binder resin of the outermost layer because the resins have insufficient mechanical strength and solvents used for dissolving the resins also dissolve the photosensitive layer. It is considered that use of an alcohol-soluble CTM prevents formation of residual images.
JOP 2002-6528 discloses a photoreceptor having a photosensitive layer and a protective layer including at least one of an alkali metal element and an alkali earth metal element. It is described therein that by including such an element in the protective layer, ionic conduction property can be imparted to the protective layer, and thereby a photoreceptor which has good durability and which does not store residual charges can be provided. It is also described therein that it is possible to reduce the residual charges by including a CTM in the protective layer but the abrasion resistance of the protective layer deteriorates.
(2) Improvement of Photosensitive Layer
JOP 2000-75521 discloses a photoreceptor including at least one of a chlorogarium phthalocyanine compound and a hydroxygarium phthalocyanine compound as a CGM and a CTM having a hydrazone skeleton. It is described in the publication that the combination of the specific CGM and CTM can diminish the transfer memory property and photo-memory property of the photoreceptor.
JOP 2000-105478 discloses a photoreceptor having a photosensitive layer including an azo pigment which is for use in an electrophotographic image forming apparatus using a laser diode emitting light with relatively short wavelength of from 380 to 500 nm as image writing light. It is considered that the azo pigments used therein have relatively weak photo-memory property compared to α-titanylphthalocyanine.
JOP 2001-305762 discloses a photoreceptor including a CGM and a CTM, wherein the CTM includes a first compound having a polarizability greater than 70 Å which is calculated by structure optimizing calculation using semiempirical molecular orbital calculation using PM3 parameter and having a dipole moment less than 1.8D which is calculated by the structure optimizing calculation, and a second compound having 50% transmittance at a longer wavelength than the first compound. It is described therein that the second compound absorbs extra light irradiating the photoreceptor, and thereby the photo-memory property of the photoreceptor can be diminished.
(3) Improvement of CTL
JOP 7-92701 discloses a multi-layered photoreceptor in which an oxytitanium phthalocaynine is included in the CGL and at least two kinds of CTMs are included in the CTL, wherein the difference in oxidation potential between the at least two kinds of CTMs is not greater than 0.04 V. It is considered that by using CTMs having almost the same energy level, hopping of the charge carriers between the CTMs can easily occur and the chance of trapping of charge carriers by the CTMs can be decreased, thereby decreasing the quantity of electrons excited by reverse charging performed by a transfer device, resulting in prevention of occurrence of the residual image problem.
JOP 08-152721 discloses a photoreceptor which is used for back-lighting type high speed image forming apparatus in which the exposure-development interval is from 10 to 150 msec, wherein the CTL of the photoreceptor has a charge mobility not less than 1×10−6 cm2/V·sec at an electric field of 2×106 V/cm. It is described therein that when a photoreceptor has a low dynamic photosensitivity, the latent image formation cannot be completed before the start of the developing operation and thereby the potential of the former image portions is increased after repeated use; but by using the technique mentioned above, the dynamic photosensitivity can be improved and thereby the residual image problem can be solved.
JOP 10-177262 discloses a photoreceptor which is for use in electrophotographic image forming apparatus having an intermediate transfer medium and which has a CGL including a phthalocyanine compound and a CTL including a compound selected from triphenylamine compounds and N,N,N′,N′-tetraphenylbenzidine compounds. It is considered that the effect is produced by the combination of the specific compounds.
(4) Improvement of CGL
JOP 06-313972 discloses a photoreceptor in which the thickness of the CGL is increased so as to be not less than 25 μm or the content of a CGM in the CGL is increased so as to be not less than 50% by weight so that a number of charge carriers are trapped in the CGL, to make the resultant ghost image invisible.
JOP 10-69104 discloses a multi-layered photoreceptor having a CGL including a triarylamine compound having a xylyl group. It is described in the publication that a barrier to carrier transportation is formed at the interface between the CGL and CTL, and charge carriers are trapped thereby. Since the trapped carriers decrease the space electric field in the CGL, the potential of a half-tone image portion is not decreased, and thereby a residual image is formed at the portion. By including a CTM (i.e., a triarylamine compound having a xylyl group), the generated carriers are rapidly injected into the CTL and transported therethrough, and thereby accumulation of trapped carriers (i.e., occurrence of the residual image problem) can be prevented.
JOP 10-186696 discloses a photoreceptor having an electroconductive substrate and at least a photosensitive layer and a protective layer located overlying the substrate in this order, wherein the photosensitive layer includes oxytitaniumphthalocyanine having an CuKα X-ray diffraction spectrum in which strong peaks are observed at Bragg (2θ) angles of 9.5°, 24.1° and 27.3°. It is considered that the effect can be produced by the specific compound.
JOP 2002-107972 discloses a photoreceptor having a CGL including a hydroxygarium phthalocyanine compound and a butyral resin which serves as the binder resin and which has an acetal group, an acetyl group and a hydroxyl group, wherein the butyral resin has a butyralation degree not less than 62% by mole, a weight average molecular weight (Mw) not less than 2.0×105 and a number average molecular weight not less than 5.0×104. It is considered that the number of photo-carriers can be reduced by the specific polyvinyl butyral, and thereby occurrence of the residual image problem can be prevented.
(5) Improvement by Matching CGL with CTL
JOP 07-43920 discloses a multi-layered photoreceptor in which a specific azo pigment is used for the CGL and a CTM having a fluorenone skeleton is used for the CTL. It is considered that addition of the specific compounds prevents the photoreceptor from suffering light fatigue.
JOP 09-211876 discloses a negative polarity-type photoreceptor having a high γ property, in which a CGL including a phthalocyanine compound and a p-type CTL including a material selected from the group consisting of inorganic p-type semiconductors and particulate t-Se and charge transport polymers. It is described therein that the p-type CTL is characterized by including no positive hole transport material and thereby diffusion of a positive hole transport material into the CGL can be prevented. Therefore, trapping caused by the phthalocyanine pigment can be prevented and thereby the residual image-problem can be prevented.
(6) Improvement of Undercoat Layer
JOP 08-22136 discloses a photoreceptor having an undercoat layer including a silane coupling agent and an inorganic filler. It is described therein that by forming such an undercoat layer, charges to be flown to the substrate are smoothly flown to the substrate, and thereby occurrence of the residual image problem can be prevented.
In addition, JOP 11-184127 discloses a photoreceptor having an undercoat layer including a resin having a specific polyamide acid or polyamide acid ester structure or a polyimide structure, and a resin having a cyanoethyl group. It is considered that by using such resins, the photoreceptor is prevented from suffering light fatigue.
JOP 2000-112162 discloses a photoreceptor having an undercoat layer including a crosslinking resin which hardly changes its resistance even when the environmental humidity changes. It is described therein that JOP 08-146639 discloses an undercoat layer including a polycyclic quinone, perylene, etc.; JOP 10-73942 discloses an undercoat layer including a metallocene compound, an electron accepting compound and a melamine resin; JOP 08-22136 discloses an undercoat layer including a particulate metal oxide and a silane coupling agent; and JOP 09-258469 discloses an undercoat layer including a particulate metal oxide having a surface treated with a silane coupling agent.
It is described therein that in a high sensitive photoreceptor including oxytitaniumphthalocyanine in its CGL, a large number of molecules and carriers are excited, and therefore there are a large number of molecules which do not cause charge separation and in addition a large number of electrons and holes tend to remain in the photoreceptor in an electrophotographic process in which charging and light irradiating are repeated.
In attempting to solve the problem, the publication (JOP 2000-112162) proposes to use a combination of a polyamide resin and a zirconium compound or a combination of a polyamide resin, a zirconium alkoxide and a diketone compound such as acetyl acetone for the undercoat layer. In addition, JOP 2001-51438 proposes to use a combination of a cellulose resin, a zirconium compound, a zirconium alkoxide, and a diketone compound for the undercoat layer.
JOP 2001-305763 discloses a photoreceptor having an undercoat layer, and including a CGM and a CTM, wherein the CTM includes a material having a polarizability greater than 70 Å which is calculated by structure optimizing calculation using semiempirical molecular orbital calculation using PM3 parameter and having a dipole moment less than 1.8D which is calculated by the structure optimizing calculation, and a specific arylamine compound, wherein the undercoat layer includes a particulate titanium oxide treated with an organic silicon compound and a polyaminde having a specific diamine component as a constituent. It is considered that by forming such an undercoat layer, the carriers remaining in the photosensitive layer can be easily transported.
JOP 2002-107983 discloses a system in which the undercoat layer of the photoreceptor has a volume average resistivity of from 1010 to 1012 Ω·cm, the CTL thereof has a thickness not greater than 18 μm and the image forming apparatus does not include a discharger. It is considered that by not using a discharger, the photoreceptor is prevented from suffering light fatigue, and since the undercoat layer has a proper resistance, injection of charges from the substrate to the photosensitive layer can be suppressed, resulting in prevention of accumulation of space charges in the photoreceptor.
(7) Addition of Additives
JOP 10-177261 discloses a photoreceptor for use in an electrophotographic image forming apparatus having an intermediate transfer medium, wherein the photoreceptor has a CGL including a phthalocyanine compound and an outermost layer including a material having a hindered phenol structure. It is considered that the effect is produced by the specific material.
JOP 2000-292946 discloses a photoreceptor having a CGL including a phthalocyanine pigment and a dithiobenzyl compound. It is described therein that by using such materials, the photo-memory property of the photoreceptor can be diminished and thereby occurrence of the posi-ghost problem can be prevented.
(8) Improvement in Image Forming Method
JOP 07-13374 proposes a technique in that the photoreceptor used is sometimes charged reversely so as to have a reverse (positive) charge, and then allowed to settle.
In a photoreceptor having a high sensitive CTL, a large number of charge carriers are induced by light irradiation. In this case, electrons whose number is the same as that of the holes injected to the CTL are formed. If the electrons are not discharged to the substrate, the electrons remain in the CGL and thereby the residual image problem tends to occur. When such a photoreceptor is reversely (i.e., positively) charged, electrons are injected from the substrate and electron traps are formed in the CGL. When light irradiation is performed on such a photoreceptor, difference in the number of electron traps between the lighted portions and non-lighted portions is little, and thereby the ghost image becomes invisible.
JOP 07-44065 discloses a technique in that a DC voltage overlapped with an AC voltage is applied to the substrate of the photoreceptor. By applying a reverse bias to the substrate, electrons trapped in the CGL can be discharged therefrom. By overlapping an AC voltage, the electric current can be increased and thereby the reverse charge bias effect can be accelerated.
JOP 10-123802 discloses a technique in that charging (not main charging) is performed on the multi-layered photoreceptor having a CGL including a phthalocyanine compound and then light discharging is performed thereon, wherein the main charging is performed thereon if the predetermined portion of the photoreceptor reaches the main charging portion. It is described therein that by performing such an image forming method, the photoreceptor is charged after the space charges in the photoreceptor are released therefrom, and thereby occurrence of the residual image problem can be prevented.
JOP 10-123855 discloses a technique in that a controller is provided in an image forming apparatus, which controls the transfer current flowing from a transfer device to the multi-layered photoreceptor used, wherein the photoreceptor has a CGL including a phthalocyanine compound. It is described therein that the greater the transfer current, the more conspicuously a negative residual image is formed. The reason is considered as follows. When an image is transferred, holes are injected into non-lighted portions of the photoreceptor and the holes are trapped at the interface of the CGL or the CTL on the substrate side. The trapped holes are released in the next charging process, and thereby the dark decay is enhanced (i.e., apparent sensitization), resulting in occurrence of formation of a negative residual image. Therefore, by controlling the transfer current, the number of charge carriers injected into the photoreceptor can also be controlled and occurrence of the residual image problem can be prevented.
JOP 2000-231246 proposes a technique in that the wavelength of the image writing light and the discharging light are determined depending on the ratio of photo-memory property before charging to the photosensitivity of the photoreceptor.
JOP 10-123856 proposes a technique in that light irradiation is performed on a photoreceptor having a CGL including a phthalocyanine compound before the transfer process to decrease the potential of the non-lighted portion to ⅓ of the potential, in order to prevent occurrence of the residual image problem. It is considered that by performing such irradiation, the difference in potential between the lighted portion and the non-lighted portion decreases and thereby the residual image becomes invisible.
JOP 10-246997 discloses a technique in that in an image forming apparatus using a photoreceptor having a photosensitive layer and a protective layer including a light-curable acrylic resin, a humidity sensor is provided in a vicinity of the photoreceptor to change the current of the AC component of the voltage applied by the charger depending on the humidity. It is described therein that by using such a technique, chance of formation of blurred images can be decreased. In addition, it is described therein that the photo-memory property of the photoreceptor is weakened using the technique, but the mechanism thereof is not described therein.
JOP 2001-117244 discloses a technique in that in order to prevent formation of ghost images when a S-form photoreceptor is used, the period of half-decay of the potential on the photoreceptor in light irradiation, which period is determined using a Xerographic Time Of Flight (TOF) method, is controlled so as to be not greater than 1/10 of the exposure-development interval between the light irradiation process and the following development process.
As described in numbered paragraph (6) above, JOP 2002-107983 discloses a technique in that by not using a discharger, the photoreceptor is prevented from suffering light fatigue.
JOP 2002-123067 discloses a technique in that the photoreceptor and charging conditions are controlled so as to satisfy the following relationship:|(V1−V2)/VH|<0.020wherein VH represents the potential of the charged photoreceptor; V1 represents the potential of the photoreceptor after a dark decay for a time of 10T, wherein T represents the charge-exposure interval; and V2 represents the potential of the photoreceptor after a dark decay for a time of 10 T, which photoreceptor is charged again after one cycle of charging followed by light irradiation has been completed.
Specifically, a technique is described therein that the process speed is increased or the photoreceptor is charged so as to have a relatively low potential.
As a result of the present inventor's experiments in which these photoreceptors and techniques are evaluated, it is found that the photoreceptors cannot be satisfactorily used as a photoreceptor which has good durability and can produce high quality images at a high speed, and by using the techniques, the resultant image forming apparatus cannot be satisfactorily used as an apparatus which can produce high quality images at a high speed.
In addition, it is discovered that when a protective layer is formed on a commercialized photoreceptor which does not cause the residual image problem, the resultant photoreceptor causes the residual image problem. Therefore the cause for the problem lies in the protective layer.
Accordingly, it is considered that even if the techniques mentioned above in numbered paragraphs (2) to (7) are applied to a photoreceptor having a protective layer, the residual image improving effect is little. In addition, if the technique mentioned above in numbered paragraph (8) is applied, the resultant image forming apparatus has one or more of the following drawbacks:    (A) all the image qualities are not high;    (B) the image forming apparatus becomes large in size; and    (C) the print costs increase.
Namely, the apparatus cannot fulfill the user's needs, and therefore the apparatus cannot be commercialized.
Further, the techniques mentioned above in numbered paragraph (1) relate to photoreceptors having no protective layer, and therefore even when the techniques are applied to photoreceptors having a protective layer, the expected effect cannot be produced. When these techniques are applied, there is a case where the effect can be hardly produced depending on the thickness of the outermost layer or the like factors of the photoreceptor. In addition, there is a case where the effects produced by forming a protective layer (such as impartment of high durability, good cleanability and good transferability) wear off when such techniques are applied.
Thus, the residual image problem cannot be fully solved.
Because of these reasons, a need exists for an electrophotographic photoreceptor which can produce high quality images at a high speed without causing the residual image problem and which has good durability, good cleanability and good transferability.